The present invention relates to power supply regulation, and particularly relates to power converters employing ripple-mode regulation.
Advances in one area of technology often require commensurate advances in supporting technologies to realize the full benefit of the advance. For example, observers of the microprocessor industry are familiar with xe2x80x9cMoore""s Law,xe2x80x9d which posits that the complexity of semiconductor devices doubles every two years. Microprocessor development arguably represents the most dramatic illustration of Moore""s Law. Pioneering microprocessors released in the 1970""s operated at clock speeds well under 500 KHz, and included fewer than five thousand transistors. Modern microprocessors operate at clock speeds in excess of 1 GHz and include millions of transistors. Exploiting these dramatic gains required advances in a host of supporting technologies, from advances in memory technology and circuit fabrication, to advances in power supply design.
Indeed, modern microprocessors could not provide their dramatic performance gains absent today""s sophisticated power supplies. For example, high-end microprocessors can consume in excess of 80 Watts of power and operate at 2 VDC or less. These requirements translate into power supply output current requirements in excess of 40 Amps, yet the power supply must maintain tight output voltage regulation, even when faced with dramatic step changes in output current. In general, modern electronic systems require responsive power supplies capable of providing relatively clean power at well-controlled voltages, over a wide range of quickly changing load conditions.
Linear regulators are responsive, accurate, and essentially noiseless. Such regulators operate using controlled impedance, typically implemented as a pass transistor, to create a voltage drop across the controlled impedance such that the input voltage minus the drop equals the desired output voltage. With proper use of load capacitors, linear regulators offer good line and load regulation performance, with virtually no noise problems. However, linear regulators are inefficient when required to regulate to an output voltage significantly below their input voltage. Because of the high currents required by modern electronic systems, main power supplies often operate at 12 VDC or 24 VDC. Regulating such primary voltages down to 2 VDC, or even 5 VDC, for high-current loads is impractical using linear regulation.
Switch-mode power supplies offer significant efficiency advantages compared to linear regulation, and avoid most of the power dissipation problems associated with linear regulators. Switch-mode power supplies operate, as their name suggests, by switching some type of reactive element in and out of a supply path to effect output voltage control. The reactive elements may be one or more capacitors, such as in a charge-pump type switcher. However, inductor-based switch-mode power supplies are more common in high-current, high-performance power supply applications. Typically, an inductor is switch-connected to a voltage source at one end, and to an output load at the other end. An output capacitor sits in parallel with the output load. A switch controller rapidly connects and disconnects the inductor to the voltage supply to regulate the load voltage. The output capacitor serves as a low-impedance current source to the load, and helps smooth the output voltage of the power supply.
The switch controller in a switch-mode power supply requires some form of feedback to effect closed loop voltage regulation on the load. Many different regulation topologies exist, including voltage-mode feedback and current-mode feedback. In both voltage-mode and current mode feedback, an error amplifier typically generates a control signal by amplifying a difference between a feedback signal and a reference signal. This error amplification can reduce the bandwidth of the feedback loop, diminishing the switch controller""s ability to respond to highly dynamic load changes, as are common with microprocessors and other high-performance electronic circuits.
Ripple-mode regulators offer greater responsiveness to load dynamics by employing a high-speed, comparator-based feedback loop. As the name implies, ripple-mode regulators regulate their output voltage based on the ripple component in the output signal. Because of their switching action, all switch-mode regulators generate an output ripple current through the switched output inductor, or inductors in a multiphase regulator. This current ripple manifests itself as an output voltage ripple due, principally, to the equivalent series resistance (ESR) in the output capacitors placed in parallel with the load. Of course, printed circuit board (PCB) trace resistance and other effects contribute to output voltage ripple.
Hysteretic controllers and constant on-time controllers are two examples of ripple-mode voltage regulators. A hysteretic controller uses a comparator to compare the output voltage being regulated, including ripple, to a hysteresis control band. Above an upper hysteresis limit, the hysteretic controller switches its associated output inductor(s) low, and below a lower hysteresis limit the hysteretic controller switches the output inductor(s) high. Constant on-time controllers operate similar to hysteretic controllers, but typically switch their output inductor(s) high for a fixed time when the output ripple falls below a single reference point.
While output ripple is useful in output voltage regulation, it is undesirable in terms of output signal noise and load voltage limits. Indeed, the desire to minimize output ripple has lead capacitor manufacturers to find new ways to reduce capacitor ESR, a chief cause of output ripple. Lowering output capacitor ESR can significantly lower the output ripple signal. Low ripple serves the interests of noise minimization and reduced load voltage variation, but makes ripple-mode regulation more challenging. Below a certain magnitude, the ripple signal becomes problematic because of noise issues and reduced comparator voltage differentials.
A virtual ripple generator provides a regulator feedback signal that includes a generated ripple component of arbitrary magnitude. The generated ripple component is synchronized to the inductor switching actions of a switched-mode voltage regulator that uses the regulator feedback signal for output voltage regulation. The virtual ripple generator forms the regulator feedback signal by combining an output feedback signal with the generated ripple component. The output feedback signal reflects the actual regulator output signal, taken at the load for example. While this output feedback signal may include whatever actual output ripple is present in the regulator""s output signal, the generated virtual ripple component is independent from actual output ripple. Similarly, any DC gain applied to the output feedback signal is independent from virtual ripple gain. The voltage regulator bases steady-state regulation on ripple in the regulator feedback signal, which includes generated ripple and actual ESR-induced output ripple. However, the voltage regulator remains responsive to transient changes in output loading, as reflected by sudden changes in the magnitude of the output feedback component of the regulator feedback signal.
The virtual ripple generator includes one or more ramp generators synchronized with the inductor switching operations of the associated voltage regulator. Nominally, the virtual ripple generator includes a ramp generator for each regulator output phase, with each ramp generator synchronized to the switching of its corresponding regulator output phase. Thus, the virtual ripple generator offers straightforward scaling for use with multiphase voltage regulators. However, the number of ramp generators and the synchronization details may vary as needed in both single- and multi-phase regulator applications. In both single, and multiphase regulation applications, the regulator has the advantage of basing phase switching on generated ripple components, rather than relying on noise-prone actual output ripple. In multiphase applications in particular, this benefit allows the regulator to maintain precise switching phase relationships between its output phases.
The virtual ripple generator can be implemented as a stand-alone circuit, or integrated into a voltage regulator. Depending upon the particular application, operating characteristics of the virtual ripple generator, such as virtual ripple magnitude, may be fixed or adjustable. Further, the virtual ripple generator may incorporate enhancements such as droop compensation. With droop compensation, the output feedback signal is adjusted in proportion to voltage regulator output current, making an offset of the regulator feedback signal provided by the virtual ripple generator responsive to output load current. This allows the voltage regulator to implement output voltage droop under high load current conditions, which can prevent undesirable voltage overshoots from occurring when the high-current conditions is suddenly relieved.